it really depends on what you are gona use the server for
ive ran servers from 386s 66mhz, 16mb ram, 1 floppy disc, to serve some static webpages
to
quad xeons, 16GB ram, 400GB hd.
Generally a p2 and above should do just fine for just a web server and a database server.
I suppose you already have internet so you would probably need a switch, a few network cables and the actual server box.
If the server is really important you should really get an UPS too.
oh yes, as Rhun said, definatelly linux/bsd